This application relates to optical transmission of information by modulating a driving current to a semiconductor laser, and more particularly, to linearization techniques and devices for reducing distortions in optical signals from modulated semiconductor lasers in optical transmitters and fiber transmission systems.
Optical waves can be modulated to carry and transmit information in various optical communications applications including CATV systems. Modulation of an optical wave may be achieved by directly modulating the driving current to a semiconductor laser such as a diode laser to produce a modulated optical output. RF signals, for example, may be superimposed on the driving current and thus be carried by the modulated optical output from the laser.
Such direct modulation of semiconductor lasers, however, is known to generate nonlinear characteristics in the modulated optical signals. The nonlinear characteristics can distort the information-carrying signals and hence are undesirable. This need for high linearity in microwave optical transmission systems is well recognized. The inherent linearity of most commercial laser diodes and integrated optical modulators is insufficient for the most demanding analog transmission applications, such as CATV signal distribution. In these cases, additional fiber optic link linearization techniques must be employed to suppress nonlinear distortion introduced principally by the optical transmitter.
One approach for linearizing the microwave signals transmitted over optical fibers uses electrical pre-distortion circuits. The electrical drive signal to the optical transmitter is pre-distorted to substantially compensate or cancel the nonlinearities introduced by the optical modulation process. Both the second order and third order nonlinear distortion characteristics can be improved through this process. See, e.g., U.S. Pat. No. 4,992,754 to Blauvelt et al.
Another approach uses an optical Fabry-Perot interferometric device in the optical path of the output beam of a modulated semiconductor laser to introduce optical distortions that reduce or cancel the nonlinearities in the modulated optical signal. See, e.g., U.S. Pat. No. 5,003,546 to Lidgard et al.